Axial piston pumps include an enclosure having an end cover and a housing portion that connects to the end cover. The end cover seals an internal sump of the pump and includes porting for the pump and includes a charge pump running surface. The porting commonly includes (i) an inlet port for providing a fluid inlet to the charge pump; (ii) internal porting from the charge pump to the enclosed axial piston pump; (iii) system ports (commonly referred to as A and B ports) for connection to a hydraulic motor; (iv) valving ports for receiving valves (such as check valve, pressure relief valve, or a combination valve) for preventing pressure spikes and for charging the hydrostatic circuit; (v) optionally a bypass valve port is provided for receiving a bypass valve that may be opened for enabling free flow between the system ports by bypassing the axial piston pump; (vi) optionally a case drain for connection to the internal sump of the hydraulic pump may be formed in the end cover. An end cover having such porting is shown and described in U.S. Pat. No. 6,332,393, hereby incorporated by reference.
The valving for the hydraulic pump may include an orifice. The orifice commonly is located in the valve seat of the check, pressure relief, or combination valve or in the end of the bypass valve. U.S. Pat. No. 6,332,393 at FIG. 15 discloses the orifice in the bypass valve.
Fluid flows through the orifice so as to soften a transition of the vehicle from a stationary position to a forward or reverse motion. When a swash plate of the pump is angled slightly relative to a neutral or zero position, flow generated by the pump passes through the orifice from a high pressure port to a lower pressure cavity (such as the low pressure port). As a result, a vehicle operated by pumps with such orifices is not set into motion as a result of the slight angle of the swash plate.
Problems with the orifices in the valves include the fact that the valves may be accidentally replaced by valves without orifices and that the special orifice valves are expensive.